More than a year into the COVID-19 pandemic, many questions still remain about immunity from prior infections and vaccinations.
For instance, how long can immunity last? Do people need to be fully vaccinated if they were previously infected with COVID-19? Do mild and severe infections differ when it comes to buildup of antibodies and levels of disease protection? How likely is it for someone to get reinfected?
Health experts are still looking into many of these questions, but offer some insight into what they have learned.
“Reinfection is relatively rare in people who have recovered from COVID-19, but it can occur. I suspect that people are protected from severe disease for a period of approximately a year or so after natural infection although they may be able to contract mild or asymptomatic infections,” said Dr. Amesh Adalja, a senior scholar for Johns Hopkins Center for Health Security.
Protection after natural infection seems to last at least six to eight months, though it likely lasts even longer, said Dr. William Schaffner, a professor of infectious diseases at Vanderbilt University.
He added researchers are still studying how long immunity from infection lasts and are not quite as confident about the proposed yearlong protection time frame as they are with the shorter time periods.
Another uncertainty is how coronavirus variants impact a person’s immunity from a previous infection.
As the coronavirus mutates and more variants emerge, there is some evidence that certain variants, specifically, the beta variant from South Africa might be able to “overcome natural immunity,” Dr. Adalja said, noting that vaccination is needed to “augment immunity.”
Increasing evidence also suggests those who were naturally infected by the coronavirus might only need one dose of the two-dose coronavirus vaccines to build up enough immunity.
A recent study by the American Chemical Society found that in participants who had COVID-19 before vaccination, the first dose produced a “vigorous antibody response similar to severe natural infection,” but the second dose didn’t provide any additional increase in antibody levels.
For participants without a previous infection, one dose of either the Pfizer-BioNTech or Moderna vaccine triggered antibody responses similar to those seen after mild COVID-19 infections, while two doses triggered antibody levels comparable to those with severe infections.
The Centers for Disease Control and Prevention is actively discussing whether previously infected individuals need to be partially or fully vaccinated, and no official recommendation has been made yet, according to Dr. Schaffner.
Antibody levels after vaccines are higher than after natural infection, meaning the shots could provide more and greater protection against emerging viral variants, the infectious disease specialist noted.
“If we reason analogously from what we know from other vaccines, that higher levels of antibody are usually associated with longer duration of protection,” Dr. Schaffner said.
Lab investigators also have said that higher levels of antibodies appear to provide more secure and greater protection against variants.
“That leads to the recommendation that people who recover from Covid should nonetheless be vaccinated,” Dr. Schaffner said.
What remains unclear still is whether a severe COVID-19 infection provides more protection than a mild infection.
“It looks like people with more severe infection on average have higher levels of viral-neutralizing antibodies and the thinking is that they may be protected for longer. But I don’t know if we know that for certain,” said Dr. Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine.
“What we do know is that if you have been infected and protected and you get boosted with a vaccine that seems to give you really high levels of broad protection,” he said, citing a study from Rockefeller University.
Moving forward, Dr. Schaffner said more research is needed to learn how long protection against COVID-19, either from natural infection or vaccines, lasts and how broad that protection is.